GB787350A - Improvements in electronic gating circuits - Google Patents

Abstract

787,350. Discharge-tube gating circuits. BRITISH BROADCASTING CORPORATION. Jan. 25, 1956 [March 11, 1955], No. 7274/55. Class 40 (6). A gating circuit comprises a cathode-coupled pair of electric discharge-tubes, the anode circuit of one of which provides the output signal and is connected to the cathode circuit. The gating signal and the signal to be gated are both applied to the control grid of each tube and the effective value of the cathode impedance is reduced when the cathode voltage rises above or falls below a predetermined level so that the circuit is unbalanced and the signal to be gated appears at the output. Fig. 1 shows a gating circuit in which the signal to be gated is applied at terminal T2 and the gating signal pulses at terminal T1, the terminals being interconnected via a resistive bridge network, so that the gating pulses are applied at greater level to grid g1 than to grid g2 whilst the signal to be gated is applied at substantially equal levels to each grid. Thus when a positive gating pulse is applied at T1 the circuit is unbalanced and current flows in tube V2, the signal to be gated is superimposed on a different D.C. bias at each grid, but due to the large negative feedback the slope of each tube is the same and there is no output at T3 of the signal to be gated. When the positive gating pulse ceases, the grid voltage falls and is followed by the common cathode voltage until rectifier D conducts, thus reducing the cathode impedance and unbalancing the circuit with respect to the signals applied at T2. An output signal appears at T3 until a further positive gating pulse is applied when rectifier D is cut-off and the output signal is terminated. When the gating pulses are of opposite polarity the direction of rectifier D is reversed. Fig. 4 shows a modification in which the gating pulses are applied at B1, C1 via cathode follower tubes V3, V4 and the signal to be gated is applied at A1 via a balancing potentiometer 22 to tubes V1, V2. Normally, tubes V1, V2 are unbalanced with respect to the signal at A1 which passes to output E1. When a gating pulse is applied, the circuit is balanced and the output ceases. In a modification, Fig. 3 (not shown) two gating circuits are associated with a feedback amplifier so that when one gate is open the other is closed.